The present invention relates to single- or multi-colored shaped ceria-stabilized tetragonal zirconia polycrystalline ceramic materials, bodies, blanks and dental shaped parts, a process for their preparation, their use for the preparation of dental restoration shaped parts and also a composition which is particularly suitable for their manufacture.
In the discussion that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicants expressly reserve the right to demonstrate that such structures and/or methods do not qualify as prior art.
The use of oxide ceramics as a framework material for dental restorations has long been state of the art. This material is characterized by an excellent biocompatibility and outstanding mechanical properties. For many years it has also been widely used as an implant material and for prostheses. In the past few years, ceramics based on partially stabilized ZrO2 ceramics have been used in particular.
The shaping of these ceramics in dental engineering is typically performed by mechanical means. In particular, milling of partially sintered ceramics with CAD/CAM processing units has gained acceptance. The shrinkage which occurs during final densification of shaped bodies, going from a density of approximately 40-60% to a density of more than 95%, is taken into account during the mechanical processing. The quoted density is relative to the theoretical density.
The disadvantages of ZrO2 ceramics are the low translucency and their milky-white color. A non-colored and non-coated restoration or restoration part looks like an unnatural tooth. Coloring the ZrO2 ceramic to match the patient's situation for an aesthetic tooth reconstruction are thus essential.
A particularly great disadvantage of sintered ceramics according to the state of the art is that they do not produce blanks for CAD/CAM processing in open-pored form or in dense-sintered form which are multi-colored or have zones of different colors corresponding to the coloration of a natural tooth.
All that is known from the state of the art is a series of technical solutions for colored, not multi-colored, blanks. However, these solutions have the disadvantage that the natural tooth color, the color gradient, the polychromatism, the graduated translucency and brightness of color were not achieved. These known solutions are described as follows:
The preparation of an open-porous colored and white Y2O3-containing ZrO2 blank is achieved according to EP 1 210 054 from liquids via co-precipitation from chlorides which contain Zr, Y, Al, Ga, Ge, In, Fe, Er and Mn ions. By means of the co-precipitation and subsequent calcination, the prepared powder already contains the coloring ions before shaping. Oxides from the group Fe2O3, Er2O3 and MnO2 are selected as coloring compounds. The disadvantage of this approach is that a very costly and laborious method of the co-precipitation process with subsequent calcination must be carried out in order to obtain a colored powder. This means that this process must be carried out for every single color.
In the following disclosures monolithic ceramics are presented which each allow one specific color, thus polychromatism, is not achieved.
U.S. Pat. No. 5,263,858 (Yoshida et el.) describes the preparation of ivory-colored shaped bodies for dental applications (brackets), wherein during the preparation of the stabilized ZrO2 ceramic coloring compounds in solutions are added before the calcination, or as powdery mixtures of coloring oxides after the calcination. In order to achieve the desired ivory shade, the addition of Fe2O3, Pr6O11 and Er2O3 is necessary. However, this process has the disadvantage that it is a multi-stage process.
It is further known from the state of the art according to FR 2 781 366 to mix the coloring components with the starting powder of the ZrO2, grind and sinter jointly. Fe2O3, CeO2 and Bi2O3 are mentioned as coloring oxides.
EP 0 955 267 mentions contents of 5-49 wt.-% CeO2, whereby coloration is achieved.
For the preparation of completely cubically stabilized zirconium dioxide in an arc-furnace process, according to EP 1 076 036 B1 one or more stabilizing and coloring oxides or their precursors are added to a ZrO2 source. The coloring oxides of the elements Pr, Ce, Sm, Cd, Tb are inserted into the crystal lattice of the ZrO2 after the sintering process.
U.S. Pat. No. 5,656,564 relates to the preparation of zirconium oxide shaped bodies which contain oxides of the rare earths boron oxide, aluminum oxide and/or silicon oxide. The shaped bodies contain zirconium dioxide as a mixed phase of tetragonal and monoclinal ZrO2. Oxides of the elements Pr, Er and Yb are introduced into the sintered ceramic as coloring oxides.
Technical solutions are further known according to the state of the art which allow colored blanks to be obtained by infiltration of liquids. However, these technical solutions have the serious drawback that coloring takes place after the pre-sintering process, and thus liquids are introduced into an open-porous ceramic body. The coloring is not completely homogeneous and also a multi-coloration cannot be achieved.
Unlike sintered ceramics, such as ZrO2 and Al2O3, a process for the preparation of multi-colored glass ceramic blanks is known from DE 197 14 178 C2. However, the preparation of multi-colored ZrO2 blanks is not mentioned in this document.
A disadvantage of the state of the art is that multi-colored sintered ceramic blanks cannot be prepared. Moreover, the solutions according to the state of the art are very costly and quality problems arise. The latter applies to the infiltration technique. Due to the subsequent coloring of a partially sintered blank or of a shaped dental product only the voids (pores) between the partially sintered particles of the starting powder can be occupied by the coloring ions. As a result, only discrete areas of the surface of the particles are colored with a layer of the coloring oxides, a continuous coverage of the surface of the particles of the starting powder not being possible. A further great disadvantage with infiltration is the concentration gradient of the coloring from the outside inwards. If a porous body is introduced into the coloring solution, the starting solution releases part of the dissolved coloring ions, starting from the outside inwards, and thus the coloring solution is “depleted” of some of its coloring substances. The consequence of this is that there is a higher concentration of the coloring ions on oxides outside than in the inside of the shaped body. Furthermore, only a certain depth of penetration can be achieved by means of the infiltration technique.
Ceria-stabilized tetragonal zirconia polycrystalline (Ce-TZP) materials are well known for their considerably higher toughness and resistance to low temperature degradation, i.e., hydrothermal stability or moisture stability, in comparison to yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP), measured under similar environmental conditions. While the fracture toughness of Ce-TZP is indeed considerably greater than that of Y-TZP (maximum fracture toughness (KIC) for Y-TZP is about 10 MPa·m0.5 whereas that for Ce-TZP is about 17 MPa/m2), the attainable flexural strength is lower than the ISO 6872 minimum requirement of 800 MPa for substructures/frameworks used for multi-unit fixed partial dentures (also known as bridges with four or more units). These Class 6 fixed dental prostheses have successfully utilized Y-TZP in many applications.
Unfortunately, hydrothermal resistance of Y-TZP ceramics has always been a concern. It has been speculated that incorporation of trivalent Y ions leads to oxygen defects, which are the main cause for hydrothermal instability of the materials. However, incorporation of low amounts of alumina into Y-TZP ceramics has resulted in significant improvement of hydrothermal stability of the Y-TZP ceramics.
The best and most effective method of developing highly resistant, hydrothermally stable ZrO2 ceramics is however incorporation of Ce ions into the lattice of the ZrO2. This incorporation also provides the toughening effect resulting in higher toughness of Ce-TZP ceramics, and simultaneously, no oxygen defects allegedly associated with hydrothermal instability are developed.
U.S. Pat. No. 5,011,403 discloses orthodontic brackets made from ZrO2 having 11 to 20 wt % CeO2, preferably 14 to 17 wt % CeO2. These ceramics however are not sintered to full density and contain nanoporosity.
There are some indications in literature that Ce-TZP can have flexural strength up to about 800 MPa, but it is still not an adequate strength in comparison to Y-TZP materials which commonly exhibit flexural strength above 900 MPa and often above 1000 MPa (up to 1.5 GPa) for better products. It is not surprising that it is Y-TZP materials that have emerged as high-strength framework materials for dental prostheses (single-units up to full arch). However due to the inherent white color of Y-TZP, often the esthetics of the finished restoration are inferior to what is achievable with other all-ceramic systems. It should be noted that Ce-TZP is not pearl-white like Y-TZP and has yellowish, ivory or beige coloration.
Currently there are two predominant commercially available methods that address the stark white color of Y-TZP zirconia. In one method, the color of the zirconia is “hidden” by applying either a layer of stain or liner. The other method entails shading the zirconia by immersion in, or painting with coloring solutions while in the pre-sintered state. Coloring with a stain and/or applying a liner involves an extra fabrication step and lowers translucency. Shading with a coloring solution similarly requires the extra step of dipping or painting, and further requires extra time to dry before sintering. Also, this method is deficient since the color of the final sintered framework often is not uniform.
An alternative method is to use porous zirconia blocks that are preshaded to the desired coloration. Such blocks only need to be fired after any machining, thus eliminating the step of coloring with solutions. As the fully sintered frameworks emerge from the furnace already shaded, the stain/liner step can be eliminated. Additionally, the color of the sintered frameworks is characteristically uniform, which is another advantage over the method of using coloring solutions for shading.
A finished dental restoration should match the color of the patient's teeth, i.e., it should be “tooth colored”. The colors of human teeth appear to range from a light, almost white-tan to a light brown, and occupy a very specific color space. This color space can be described by the commonly used CIE (Commission Internationale de l'Eclariage) L*, a*, b* conventions, which represents colors in a three-dimensional Cartesian coordinate system. L*, or “value”, is a measure of luminance or lightness, and is represented on the vertical axis. The a* and b* coordinates, are a measure of chromaticity and are represented on the horizontal coordinates, with positive a* representing red, negative a* representing green, positive b* representing yellow and negative b* representing blue. U.S. Pat. No. 6,030,209, which is incorporated herein by reference, discusses the CIE L*, a*, b* color coordinates of tooth colors represented by the Vita Lumen® shade guide system manufactured by Vita Zahnfabrik. It provides the color space of tooth colors. Hereinafter, “tooth color” is taken to mean CIE L*, a*, b* color coordinates that fall within or very close to this color space. In terms of coloration, three areas can be distinguished in natural dentition: (a) the incisal area, which is the more translucent; (b) the middle section of the tooth; and (c) the cervical area, which is more chromatic and more intensively colored. Multiplicity of colors of natural dentition exclusive of incisal and cervical areas can be quantitatively described as belonging to color space delineated by L* from about 60 to about 90, a* from about −3 to about +10, and b* from about 12 to about 36.
U.S. Pat. No. 6,713,421, which is hereby incorporated by reference, appears to describe yttria-stabilized zirconia dental milling blanks that are formed with 0-1.9 wt. % coloring oxides from elements of the group Pr, Er, Fe, Co, Ni, Ti, V, Cr, Cu, Mn, with Fe2O3, Er2O3 or MnO2 preferably being used. The composition described therein includes 0.1 to 0.50 wt. % of at least one oxide of aluminum, gallium, germanium and indium for the purpose of lowering the sintering temperature and increasing stability and hydrolytic resistance in the densely sintered state. However, the addition of alumina to zirconia also often results in discrete alumina inclusions distributed throughout the microstructure. This occurs in part due to the low solubility of alumina in zirconia. Further, it presents a particular disadvantage for dental applications because alumina inclusions can lower the translucency of the zirconia since the refractive index of alumina, 1.77, differs considerably from that of tetragonal zirconia, which is 2.16. For example, alumina was added to Ce-TZP in an attempt to strengthen it resulting in very opaceous but strong Ce-TZP-Al2O3 nanocomposite material. Thus, it is desirable that dental zirconia is devoid of any alumina inclusions. A means to achieve this is to minimize, or eliminate, the alumina addition, thereby minimizing the potential for the alumina inclusions in the final microstructure.
In U.S. Pat. No. 6,713,421 the blanks are made from powders or granules that have been doped with the various oxides via a solution followed by a co-precipitation method. The cited advantage of this method is that the various oxides are distributed homogeneously throughout the powder. However, the field of dental restoratives requires many shades (for example, the Lava system offers 7 zirconia core shades and the Vita Classic system offers 16 Vita shades.). Having to prepare so many individually shaded powders or granules can be cost-prohibitive
Another disadvantage of the method set forth in U.S. Pat. No. 6,713,421 is that it requires relatively large amounts of the coloring oxides, iron oxide and erbium oxide. The examples reveal the addition of 0.2 wt. % iron oxide+0.38 erbium oxide (0.58% total) to provide color to 3Y-TZP. Although the patent does not indicate whether this results in a tooth color, it can be inferred from U.S. Pat. No. 5,219,805, which appears to disclose coloration of yttria-stabilized zirconia for dental bracket applications using combinations of Fe2O3, Er2O3, and Pr6O11, that even higher Fe2O3 and Er2O3 concentrations are necessary to achieve tooth coloration. For instance, according to the examples given in U.S. Pat. No. 5,219,805, up to 1.0 mol % Er2O3 (3.0 wt. %) additive is required to achieve dental brackets “having color tone similar to ivory-colored teeth”. Additionally, up to 0.2 mol % Fe2O3 (0.25 wt. %) is required to achieve tooth colors, which although less than the 1 mol % Er2O3 is required, it is still a considerable amount. Such significant quantities can have a negative effect on other properties of the resulting Y-TZP cores, such as on strength, Weibull modulus, hydrolytic resistance, and grain size.
Additionally, it has been observed that Er2O3 additions to 3Y-zirconia, of 0.2 wt. % or greater, result in sintered bodies that fluoresce to a dark yellow under ultraviolet (UV) lighting. This is inappropriate for a dental framework, which under UV light, ideally, should fluoresce bluish-white to mimic that of natural teeth. Less ideally, the framework should not fluoresce at all in the visible light range. In the latter case, fluorescence is typically imparted to the final restoration by the overlay porcelains. The shortcoming of an inappropriate fluorescence is overcome by the present invention.
The prior art also shows that Cr additions result in green or brown coloration. For example, U.S. Pat. No. 3,984,524 appears to describe olive coloration of cubic zirconia with the addition of 0.1 to 2 wt. % Cr2O3, U.S. Pat. No. 4,742,030 appears to describe green coloration of 5 mol % yttria-stabilized zirconia with the addition of 0.7 wt. % Cr2O3, and brown coloration with addition of 0.2 wt. % Cr2O3, respectively.
U.S. Pat. No. 5,656,564 appears to teach coloration of zirconia for dental bracket applications using combinations of Er2O3 and Pr6O11. The sintered zirconia-based ceramic is produced by a procedure generally including combining constituents in solution, precipitating, calcining, pressing, and sintering.
U.S. Pat. No. 5,011,403 appears to describe coloration of zirconia dental brackets using combinations of one or more of oxides of Fe, Ni and Mn added to a Zr-based powder.
U.S. Pat. No. 6,709,694 appears to describe the use of solutions for coloring of pre-sintered zirconia dental frameworks by immersion, painting or spraying using a metal ion coloring solution or metal complex coloring solution that is applied to a presintered ceramic, followed by sintering to form a translucent, colored dental ceramic. The claimed ions or complexes are of the rare earths elements or subgroups II and VIII, which have an action time of under two hours, and maximum pre-sintered zirconia diameter and height of 10 and 7 mm, respectively. However, this method is not ideal as the color of the final sintered frameworks often are not uniform and the process requires the extra steps of applying the solutions and drying prior to sintering.
The development of pink coloration in zirconia by Er additions is described in (i) P. Duran, P. Recio, J. R. Jurado, C. Pascual and C. Moure, “Preparation, Sintering, and Properties of Translucent Er2O3-Doped Tetragonal Zirconia,” J. Am. Ceram. Soc., vol. 72, no. 11, pp. 2088-93, 1989; and (ii) M. Yashima, T. Nagotome, T. Noma, N. Ishizawa, Y. Suzuki and M. Yoshimura, “Effect of Dopant Species on Tetragonal to Monoclinic Phase Transformation of Arc-Melted ZrO2—RO1.5 (R=Sm, Y, Er, and Sc) in Water at 200° C. and 100 MPa Pressure,” J. Am. Ceram. Soc., no. 78, no. 8, pp. 2229-93, 1989. Additions of CoO, Fe2O3 and Cr2O3 combinations to yttria-stabilized zirconia are known to impart a blue color in the final sintered zirconia bodies, as apparently described in Japanese patent publication 2,145,475. Additions of one or both of the colorants, nickel oxide and cobalt oxide, to yttria-stabilized zirconia have been shown to result in a purplish colored sintered body, as apparently described in U.S. Pat. No. 5,043,316.
Japanese patent publication 3,028,161 appears to describe the preparation of colored zirconia by the steps of: (1) mixing zircon-based pigment with partially stabilized zirconia containing Y2O3, MgO, etc., (2) molding and (3) sintering to provide a colored zirconia sintered product.
Many of the aforementioned coloring additions can negatively affect not only mechanical properties, including strength and fracture toughness, but also isotropic shrinkage and final sintered density. This can happen for a number of reasons including: (1) loss of fracture toughness from a lowering of the “transformation toughening” effect as a result of the over-stabilization of the tetragonal phase by the additive (either chemically, or by grain size reduction) thereby hindering the transformation from the metastable tetragonal phase to monoclinic phase that is necessary for the toughening to happen, (2) loss of strength due to spontaneous microcrack formation that can result if grains grow too large because of the additive, and, (3) loss of strength due to the formation of strength-limiting pores in the microstructure due to the additive. This last reason is what Shah et al. (K. C. Shah, I. Denry and J. A. Holloway, “Physical Properties of Cerium-Doped Tetragonal Zirconia,” Abstract 0080, Journal of Dental Research, Vol. 85, Special Issue A, 2006) attribute the significant loss of strength, down to 275±67 MPa, for 3Y-TZP materials that were colored using Ce salts. Additionally, they observed that strength decreased linearly with the concentration of the coloring additive, Ce.
The problem of formation of coarse pores, along with grain growth, in colored zirconia sintered compacts has also been recently recognized in JP 2005289721.
It is also important to recognize that only certain combinations of coloring agents in certain proportions will enable the matching of the color of a dental article so as to match the desired natural tooth color, e.g., A, B, C, D of the Vita™ classic shade guide and the Chromoscop® universal shade guide.